What is Open Block?

The basic token-based Absolute Block system described above has three states: Line Clear, Train on Line, and Line Closed. (Of course, on single-line systems there may be Train Coming From and Train Going To instead of Train on Line). This is also known as Three-Position Block, or Closed Block, because the fundamental assumption is that the block section is closed as the default state, and after a train has been received intact after a Line Clear, the line must go to Line Closed, and a second Line Clear is needed for any further operation. In the Open Block system, the block section reverts to Line Clear after Train on Line, after the train has been received intact. In other words, the default state of the line is open, and either station may send a train on the line without the prior permission of the other station. This, obviously, contradicts a basic principle of Absolute Block working that the station in advance must give permission for a train to enter the block section. The advantage is that some time is saved in not having to ask for and obtain Line Clear - but safety is reduced and this requires careful and vigilant working. Sometimes this is combined with Controlled Manual Block (see below). Push-button type token instruments often work in Open Block mode - either station master can obtain Line Clear directly, after the previous train has been acknowledged to have arrived intact and the instrument has been normalized.

Source - IFRCA.org